This invention relates to the preparation of ceramic materials with increased crystallinity. More specifically this invention relates to the preparation of ceramic materials with increased levels of crystalline SiC and/or Si.sub.3 N.sub.4. The ceramic materials of this invention are obtained by firing a mixture of a R.sub.3 SiNH-containing silazane polymer and certain inorganic compounds to an elevated temperature in an inert atmosphere or in a vacuum. The additives that allow for ceramic materials with increased crystallinity include iron compounds, cobalt compounds, nickel compounds, and copper compounds.
Ceramic material prepared from R.sub.3 SiNH-containing silazane polymers are known to the art. Gaul in U.S. Pat. No. 4,340,619 (issued July 20, 1982), which is hereby incorporated by reference, prepared ceramic materials by firing at elevated temperatures in an inert atmosphere a R.sub.3 SiNH-containing silazane polymer prepared by contacting and reacting, in an inert, essentially anhydrous atmosphere, chlorine-containing disilanes with disilazanes. The ceramic material of U.S. Pat. No. 4,340,619 contained crystalline SiC and/or Si.sub.3 N.sub.4 only when the R.sub.3 SiNH-containing silazane polymer was fired to at least 1600.degree. C.
Gaul in U.S. Pat. No. 4,312,970, issued Jan. 26, 1984, which is hereby incorporated by reference, obtained ceramics by firing a R.sub.3 SiNH-containing silazane polymer, prepared by contacting and reacting an organochlorosilane with a disilazane, at elevated temperatures in an inert atmosphere or in a vacuum. The crystalline content of the ceramic materials was not reported.
What is newly discovered is that certain inorganic compounds when added to R.sub.3 SiNH-containing silazane polymer prior to firing at elevated temperatures, allow for the formation of ceramic materials with increased crystallinity relative to ceramic materials fired under the same conditions without the additives. The process of this invention allows for the formation of ceramic materials with increased levels of crystalline Si.sub.3 N.sub.4 and/or SiC. The process of this invention also allows for the formation of ceramic materials containing crystalline Si.sub.3 N.sub.4 and/or SiC at lower pyrolysis temperatures.